ABSTRACT The broad long-term goal of this application is to identify innate and T helper (Th) cell transcriptomic signatures of immune response and further our understanding of sex-dependent human immune responses to two unique influenza A/H3N2 vaccines in a population of older adult males and females. We will comprehensively measure the spectrum of innate and adaptive (CD4+T helper cell) immune responses to the recently FDA- licensed MF59-adjuvanted influenza subunit vaccine (MF59Flu) and the high-dose split influenza virus vaccine (HDFlu) using a systems biology approach, combined with detailed clinical and laboratory immunophenotyping in a well-characterized cohort (65 years of age and older). Early innate immune responses are critical to the development of robust adaptive immunity that confers protection upon re-exposure to influenza. Likewise, Th responses provide T cell help essential for the establishment of protective humoral immunity. Little is known about the effect of sex on innate and Th helper responses following these influenza vaccines. In this application, we propose two parallel Specific Aims (one focused on innate immune responses and the other on CD4+Th responses to these two vaccines). Our Aims will test the following hypotheses: 1) that vaccine type (MF59Flu vs HDFlu) and/or sex are associated with variations in innate and Th cell immune response; 2) that the increased Ag dose in HDFlu results in greater activation/suppression of the genes/genesets previously associated with immune responses to standard dose influenza vaccine; 3) that the MF59 adjuvant results in activation/suppression of additional genes/genesets compared to HDFlu; 4) that transcriptomic signatures (gene expression patterns associated with immune responses) mediate the association of vaccine type (or sex) with immune outcomes; and 5) that the innate or Th cell immune outcomes and corresponding transcriptomic signatures will predict markers of humoral immunity (HAI titer and memory B cell ELISPOT response). Completion of these Specific Aims will allow us to dissect the molecular mechanisms by which MF59Flu and HDFlu enhance innate and Th immune responses in older persons at high risk of influenza disease, identify specific genesets involved in sex-based differences in immune response, and may allow the identification of new correlates of vaccine immunogenicity.